Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in publications by C. H. Bennett et al entitled “Experimental Quantum Cryptography” J. Cryptology 5: 3-28 (1992), and by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992).
The above-mentioned publications each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization or phase of single photons, and Bob randomly measures the polarization or phase of the photons. The one-way system described in the Bennett 1992 paper is based on two optical fiber Mach-Zehnder interferometers. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The signals (pulses) sent from Alice to Bob are time-multiplexed and follow different paths. As a consequence, the interferometers need to be actively stabilized to within a few tens of nanoseconds during transmission to compensate for thermal drifts.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent), which patent is incorporated herein by reference, discloses a so-called “two-way” QKD system that is autocompensated for polarization and thermal variations. Thus, the two-way QKD system of the '234 patent is less susceptible to environmental effects than a one-way system.
The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33. As described therein, during the QKD process, Alice uses a true random number generator (TRNG) to generate a random bit for the basis (“basis bit”) and a random bit for the key (“key bit”) to create a qubit (e.g., using polarization or phase encoding) and sends this qubit to Bob. The collection of exchanged gubits is called the “raw key.” Alice and Bob then use a public channel compare the bases used to measure the qubits and keep only those bits having the same basis. This collection of bits is called the “sifted key.
While QKD is theoretically secure, the practical implementation of QKD allows for several ways for an eavesdropper to get information about the key bits. For example, to encode the value of a key bit on a photon one needs fast electronics, which produce electromagnetic radiation. This radiation can be measured by an eavesdropper in a so-called “side channel attack.” For phase-encoded QKD, this may be a serious problem, since phase modulators can actually produce enough measurable electromagnetic (EM) radiation. Second, an eavesdropper might get partial information on the key by monitoring transmission in the fiber. This is possible when multi-photon pulses are produced by a weak coherent source. An eavesdropper can measure such pulses without introducing errors in the transmission. Third, an eavesdropper may be able to launch a so called “Trojan horse attack” on Alice with a well-timed probing pulse in order to obtain information about the state of the phase modulator.